Northumberland has sent a message to Hotspur, Worcester, and Douglas:
he's sick and cannot come to the war. Northumberland "is remembered as
the man who in the abdication scene kicked Richard when he was down.
Confirming the old proverb about the bully, he is the archcoward of
Henry IV" (Goddard, I 166). So he won't be leading his
army to Shrewsbury. Hotspur still wants a fight though: "I rather of
his absence make this use: / It lends a lustre and more great opinion,
/ A larger dare to our great enterprise" (IV.i.76-78).

Sir Richard Vernon brings news of the approaching royal armies,
including that of Prince Hal, whom is again called "madcap" (IV.i.95).
And though the Prince is impressing all, surprisingly, Hotspur is revved
and wants to battle him specifically: "Harry to Harry shall, hot horse
to horse, / Meet and ne'er part till one drop down a cor[p]se"
(IV.i.122-123). Vernon adds the additional bad news that Glendower
seems to be wimping out, claiming to need more time to gather his
forces. Hotspur's vaunting enthusiasm for battle under these increasingly
grim conditions begins to seem deluded, or even pathological.

The attention to Shrewsbury may have partly been because the
Earl of Shrewsbury served as custodian of Mary Stuart; she was
imprisoned in his castle for most of her time in England (Clark
692). A descendant of Northumberland -- Henry Percy, 9th Earl of
Northumberland -- was a contemporary of Shakespeare and was
considered as a possible marriage candidate for Elizabeth Vere,
Oxford's oldest daughter, around 1592 (Farina 118). Meanwhile,
the 11th Earl, so prominent in Famous Victories, has
all but disappeared. Apparently, for Oxford in 1586 "the terms
of the grant had stipulated stringent anonymity" (Ogburn and
Ogburn 713). The necessary expertise in horsemanship in this scene
(IV.i.104-110) has also supported the questioning of traditional
authorship (Ogburn and Ogburn 722).

SCENE ii

Falstaff and Bardolph lead an outfit of would-be soldiers near
Coventry. Falstaff sponges off Bardolph, obviously, wanting another
bottle of sack, and he comments to himself at length regarding the
ragtag state of his men. Hal and Westmoreland enter and remark on
the state of these troops: "I did never see such pitiful rascals"
(IV.ii.64). Falstaff responds, "Tut, tut, good enough to toss,
food for powder, food for powder [cannon-fodder]; they'll fill
a pit as well as better" (IV.ii.65-67). Onward towards the field
of battle they advance, Falstaff in no particular hurry. "To the
latter end of a fray and the beginning of a feast / Fits a dull
fighter and a keen guest" (IV.ii.79-80).

SCENE iii

Hotspur and Douglas want to fight; Worcester and Vernon are
reluctant. Hotspurs seems a bit desperate in focusing on the
advantage they have in terms only of rested horses. Blunt
offers a deal from the King. Hotspur rants about Henry's
ingratitude to the people who helped him to the throne:

The King is kind, and well we know the King
Knows at what time to promise, when to pay.
My father and my uncle and myself
Did gve him that same royalty he wears....
(IV.iii.52-55)

Hotspur rails at length regarding Henry IV's treacheries and
unworthiness. But between this and Worcester's explanation in
the first scene of the last act, we finally get the full case
of complaint against Henry IV. They'll respond formally to Henry
tomorrow morning.

SCENE iv

The Archbishop of York has instructions delivered to his allies. He
knows that Northumberland, Glendower, and Mortimer have bailed, thinks
Hotspur will be beaten, and fears Henry's retaliation for his own part
in the rebellion. This material will matter more in Henry IV,
Part 2.